The practice could make it harder for photographers to claim ownership of their work

Photographers turning to social networks like Facebook and Twitter to promote their work may be losing the legal rights to their photos because the sites are deleting the images' metadata.

Those sites, in addition to Flickr, are among some of the largest social networks that delete the identifying data at various stages of the image upload and download process, according to the results of a survey released Tuesday by the International Press Telecommunications Council, a worldwide consortium of news agencies based in London.

The deletion of such data is a problem for professional photographers because the metadata often includes key information such as who owns the image's copyright, the photographer's name, captions and other descriptive data.

If these data disappear, the floodgates are open for unauthorized use of photographers' photos, IPTC claims.

"A social networking site is only as good as the information its members choose to share," said Michael Steidl, IPTC managing director, in a statement. "If users provide rights data and descriptions within their images, these data shouldn't be removed without their knowledge," he said.

Some sites, for instance, may have policies in place requiring that they remove the information to speed up the download time for viewing the images, which "may make sense for the service, but not for users of their service who are interested in protecting their intellectual property," said a post on Controlledvocabulary.com, an image rights group that assisted in conducting the survey.

On both Facebook and Flickr, survey testers found that while the metadata was more or less retained upon upload, it disappeared when images are either saved locally or downloaded into other photo management software.

Image metadata was also deleted when saving a photograph locally after posting it on Twitter, the study showed.

Google+ and Tumblr, on the other hand, performed better on the test. The metadata for images hosted on Google+ remained intact for every test that was performed.

Image metadata has legal implications too in light of proposed "orphan works" legislation, which would make it easier for other people to use and profit from photographs that don't have certain identifying information, such as contact information or copyrights, attached to them.

The orphan works proposal, which is being deliberated by Congress as part of U.S. copyright law, "shows just how dangerous this can be when it comes to copyright because it essentially creates immediately 'orphaned' images that can then be used for free because it's no longer possible to contact the photographer and get the proper permission," said Grover Sanschagrin, co-founder of PhotoShelter, an online image archive and distribution system for serious and professional photographers.

Still, the notion that social networks may be abusing image metadata is not altogether new, and photographers should know what they're getting into when they use free sites like Facebook to post their photos, some say.

"Professional photographers know that when you use free social media you're at their mercy, so to rely solely on Facebook or Twitter for something like a portfolio when you use nothing else and then complain about it kind of makes you look foolish," said Megan McGory, a freelance news photographer based in Lisbon, Connecticut.

Deleting metadata could be an issue, on the other hand, if someone is trying to track cases of copyright infringement or even stolen cameras, McGory added. "The camera's serial number is in the metadata and you can find sites using images taken with your camera that way," she said.

Facebook, Twitter and Flickr did not immediately respond to requests for comment on this story.